The objective of the proposed research is to determine the role played by amino acid transport in the metabolism of cells, and the importance of changes in this transport produced by the action of specific hormones. Control of amino acid transport is one way in which hormones and other factors may control metabolism. Our earlier work with isolated rat diaphragm has shown that hormones may alter only one of the amino acid transport stems in cells, so that entry of only some amino acids may be modified extensively in any specific situation. Cellular levels of some amino acids and their metabolites may therefore be modified by a given agent, while others are not. The altered amino acid or metabolite levels in the appropriate cells may be important in determining the subsequent alterations in metabolism. In the present proposal, the effects of hormones on amino acid transport will be studied in the developing rat brain in vitro an in vivo. The investigation will include 1) determination of the amino acid transport systems present in the developing brain compared with the mature brain, especially to determine the qualitative and quantitative changes that take place in these systems during the period of most rapid development of brain structure and mental capacity; 2) determination of the changes in amino acid transport systems produced by specific hormones known to alter brain developmet. The first studies will be made with thyroid hormone, and then growth hormone, glucagon and epinephrine, because of the importance of these hormones in amino acid transport and their apparent relationship to some aspect of brain metabolism and/or development.